Electron Beam Instability and Isodose Asymmetry Associated with a 35 MeV Medical Betatron

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Abstract
Initial dosimetry measurements on a Brown-Boveri 35 Mev betatron indicated poor reproducibility of electron beam isodose curves. An investigation showed that the beam position and shape depended strongly upon a given combination of electron energy and extraction voltage. Small changes in extraction voltages or small drifts in energy produced large asymmetries in the radiation field. Under normal operating conditions the asymmetries were large enough to present serious clinical problems. A special device subsequently developed by the manufacturers has eliminated these difficulties. To monitor beam symmetry, two cylindrical ionization chambers were mounted in the fringe of the radiation field. The output of these chambers is fed subtractively to a voltmeter located on the control console, symmetry being indicated by a voltage null. An auxiliary extraction-voltage control has been incorporated in the system and symmetry is maintained by a manual adjustment of this control.

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